[Arubis]

Multimode is probably fine, provided it's OM3 or higher. You might need to think about it down the road, 5+y out. Having a spool just lying around would make it a perfectly reasonable choice. The fibre itself is a little more expensive; the optics are cheaper. But multimode does run into physical limitations a little faster, so when you decide you want to go to 100G, or 400G, or beyond, you may need to pull fresh media.

Singlemode you just never have to think about again. It's likely to be more future-proof than your actual physical walls.